:wrapper

FLI type descriptor

Summary

Allows the specification of automatic conversion functions between Lisp and an instance of an FLI type.

Package

fli

Syntax

:wrapper
foreign-type
&key
lisp-to-foreign
foreign-to-lisp

Arguments

foreign-type

The underlying type to wrap.

lisp-to-foreign

Code specifying how to convert between Lisp and the FLI.

foreign-to-lisp

Code specifying how to convert between the FLI and Lisp.

Description

The FLI
:wrapper
type allows for an extra level of conversion between Lisp and a foreign language through the FLI. With the
:wrapper
type you can specify conversion functions from and to an instance of another type. Whenever data is passed to the object, or received from the object it is passed through the conversion function. See below for an example of a use of
:wrapper
to pass values to an :int as strings, and to receive them back as strings when the pointer to the :int is dereferenced.

Example

In the following example an :int is allocated with a wrapper to allow the :int to be accessed as a string.

(setq wrap (fli:allocate-foreign-object

:type '(:wrapper :int

:lisp-to-foreign read-from-string

:foreign-to-lisp prin1-to-string)))

The object pointed to by
wrap
, although consisting of an underlying :int, is set with dereference by passing a string, which is automatically converted using the Lisp function
read-from-string
. Similarly, when
wrap
is dereferenced, the value stored as an :int is converted using
prin1-to-string
to a Lisp string, which is the returned. The following two commands demonstrate this.

(setf (fli:dereference wrap) "#x100")(fli:dereference wrap)

The first command sets the value stored at
wrap
to be 256 (100 in hex), by passing a string to it. The second command dereferences the value at
wrap
, but returns it as a string. The pointer
wrap
can be coerced to return the value as an actual :int as follows: